The present invention relates to a method and apparatus for directional radio communication between a first station and a second station. In particular, but not exclusively, the present invention is applicable to cellular communication networks using an antenna array in an adaptive manner, for example, space division multiple access networks.
With currently implemented cellular communication networks, a base transceiver station (BTS) transmits signals intended for a given mobile station (MS) which may be a mobile telephone, throughout a cell or a cell sector served by that base transceiver station. However, systems using adaptive antenna arrays such as space division multiple access (SDMA) systems have been proposed. In adaptive antenna array systems, the base transceiver station will not transmit signals intended for a given mobile station throughout the cell or cell sector covered by that base transceiver station but will only transmit the signal over a smaller proportion of the cell or cell sector. The direction in which a signal is transmitted to a given mobile station is usually determined in accordance with the direction from which a signal from the mobile station is received.
In some known cellular communication networks, it is common to have highly directional traffic. In other words, the base station or a mobile station will transmit far more information than it receives. For example, this can occur with packet radio networks. In packet radio networks, the data is transmitted to or from a mobile station in the form of packets. Accordingly, the time division duplex (TDD) mode has been proposed. Effectively, the same radio channel characteristic will be used for signals transmitted by the mobile station to the base transceiver station and also for those signals transmitted by the base transceiver station to the mobile station. The signals are not transmitted by the mobile station and the base transceiver station at the same time. In a time division multiple access (TDMA) system, certain of the slots in a frame will be used by a mobile station to transmit signals to a base transceiver station and the remaining slots will be used by the base transceiver station to transmit signals to the mobile station.
However, if in an adaptive antenna array system, there is highly directional traffic, regardless of whether or not the TDD mode is used, problems can occur. These problems occur if the base transceiver station is transmitting far more signals to the mobile station than the mobile station is sending to the base transceiver station. In particular, the base transceiver station needs to be able to determine the direction from which signals from a mobile station are received in order to determine the direction in which the base transceiver station should transmit signals to the mobile station. If, however, the mobile station is moving and is only transmitting signals irregularly to the base transceiver station, the base transceiver station is not able to properly track the position of the mobile station. The risk that the base transceiver station will transmit signals in the incorrect direction will increase.
This same situation also gives rise to problems in TDD modes of operation. The channel is regarded as being reciprocal. In other words, the channel behaviour in the direction of transmission of signals from the base transceiver station and to the mobile station are regarded as being equivalent. However, if the mobile station only infrequently transmits signals to the base transceiver station, the base transceiver station will not be able to deduce very much about the radio channel since it receives very little information from the mobile station. Accordingly, parameters of the signal transmitted by the base transceiver station, such as directional information of the signal path, which are dependent on information derived from signals received by the base transceiver station will only be updated infrequently. This can lead to a decrease in the network performance.
Reference is made to Finish Patent application No FI 941072 which is also in the name of the present applicant. This document discloses an arrangement in which the number of pilot symbols included in a channel is varied in accordance with the quality of the radio channel.
It is an aim of embodiments of the present invention to address these problems.
According to one aspect of the present invention, there is provided a method of directional radio communication between a first station and a second station, said method comprising the steps of transmitting signals from said second station to said first station via a radio channel; measuring at least one parameter indicative of the speed of change in said radio channel; determining from said at least one parameter a rate with which said second station should send signals to said first station; and causing said second station to send signals to said first station with at least said rate.
Thus, the rate at which the second station sends signals to the first station is determined from at least one parameter indicative of the speed of change of the radio channel. This improves the quality of the signals.
The method preferably includes a step of transmitting signals from the first station to the second station. More signals may be transmitted from the first station to the second station than are transmitted to the first from the second station. It is also possible that approximately the same number of signals be sent from the first station to the second station as from the second station to the first station. It is also possible that more signals may be transmitted from the second station to the first station than are transmitted from the first station to the second station.
The at least one parameter may be the speed of the second station relative to the first station and the measuring step may measure the speed of the second station relative to the first station. Alternatively or additionally, the at least one parameter may be the distance between the first and second stations and the measuring step may measure the distance between the first and second stations. Alternatively or additionally, the at least one parameter is the coherence time of the radio channel and the measuring step measures the coherence time of the channel. Alternatively or additionally, the at least one parameter may be the angular spread of the signal received by the first station from the second station and the measuring step measures said angular spread. Alternatively or additionally, said at least one parameter may be the height of an antenna array of the first station above the environment.
The rate may be proportional to (the speed of the second stationxc3x97angular spread of the signal received from the second station by the first station)÷(the distance between the first and second stationsxc3x97the coherence time of the channel). The rate is preferably higher with higher speeds of change in the channel.
The first and second stations may communicate using a time division multiple access method with frames divided into slots. The rate may be defined as one signal to be transmitted from the second station to the first station every n frames where n is an integer.
The first and second stations may use the same frequency range for the transmitted signals or alternatively may use different frequency ranges. A reference signal may be sent from the second station to the first station with the predetermined rate.
Preferably the second station is a mobile station and the first station is a base transceiver station.
According to a second aspect of the present invention, there is provided a first station for directional radio communication between with a second station, said first station comprising means for receiving signals transmitted from said second station to said first station via a radio channel; means for measuring at least one parameter indicative of the speed of change in said radio channel; means for determining from said at least one parameter a rate with which said second station should send signals to said first station; and means for transmitting said rate to said second station.